Perceptual Coherence : Hearing and Seeing

larger arrays and Baddeley and Tirpathy (1998) suggested the perception of
movement was based on the fraction of dots that moved, so that the detection
of motion appears to be a global operation on the whole pattern, and
not a local operation founded on the perceived motion of just several dots.
If the displacement is beyond d max, the target region does not appear to
move. Instead, the dots within the region appear to oscillate independently
in different directions, being the chance pairing of noncorresponding dots.
Second, there is a limit to the onset-to-onset interval. In Braddick’s experiment,
if that interval was greater than 50 ms, the perception of motion was
diminished. Moreover, there was no relationship between displacement and
time as found for the apparent movement of single figures. Third, apparent
movement does not occur if the first array is presented to one eye and the
second array is presented to the other eye. In contrast, apparent movement
of single figures will occur for the same alternating eye sequence.
I will return to the difficulties of the short-long distance distinction.
However, I do believe that there is a real distinction between short-distance
perception based on spatial correlation and long-distance perception based
on the identification and correspondence of features, and that the short-long
distinction is very similar to the pitch-feature distinction discussed for the
perception of repeated noises in chapter 4. A segment of auditory noise is
just like a random dot pattern.
Apparent Motion of Nonrigid Arrays
Perception of Motion 215
Another type of visual stimulus used to study motion detection is similar to
the seemingly unconnected movements of the fireflies. A random array of
dots is shown in the initial frame. In the next (and successive) frames, each
dot moves to a new position. The direction and distance of movement could
be randomly determined for each dot, or the direction and distance of
movement could be constrained for a subset of the dots. For example, 10%
of the dots would move vertically in each frame. The observer’s task is to
identify the direction of the coherent subset of dots.
This paradigm is similar to but more complicated than the random dot
arrays described in the previous section. In those experiments, all of the
dots that shifted from frame to frame formed a connected vertical or horizontal
rectangle so that each dot moved identically. Here, the dots constrained
to move in one direction are usually scattered throughout the entire
field, and each can move a different distance. Moreover, different dots will
move in the target direction on successive frames. Any single dot has a limited
lifetime, so that it can move in one direction only for very few steps.
This restriction disallows observers from tracking the movement of a single
dot. The movements shown in figure 5.9 illustrate these constraints. In the
two frames shown, 4 of the 10 dots move to the right. But only dots F and